1. Field of the Invention
The present invention relates to a semiconductor device including a circuit, which is formed using a thin film transistor (hereinafter referred to as a TFT), and a manufacturing method thereof. For example, the present invention relates to an electronic device provided with, as a component, an electro-optical device typified by a liquid crystal display panel or a light-emitting display device having an organic light-emitting element.
Note that in this specification, a semiconductor device refers to all types of devices which can function by utilizing semiconductor characteristics. An electro-optical device, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
In recent years, a technique for forming thin film transistors (TFTs) using a semiconductor thin film (a thickness of several tens of nm to several hundred nm) formed over a substrate having an insulating surface has been attracting attention. Thin film transistors have been widely applied to electronic devices such as ICs and electro-optical devices, and their development especially as switching elements in image display devices has been accelerated.
A thin film transistor using an amorphous semiconductor film, a thin film transistor using a polycrystalline semiconductor film, and the like are used as switching elements in image display devices.
In the case of the thin film transistor using an amorphous semiconductor film, an amorphous semiconductor film such as a hydrogenated amorphous silicon film is used; therefore, there is limitation on the process temperature, and heating at a temperature of greater than or equal to 400° C. at which hydrogen is detached from the film, intense laser beam irradiation by which surface roughness occurs due to hydrogen in the film, and the like are not performed. The hydrogenated amorphous silicon film is an amorphous silicon film whose electric characteristics are improved by bonding of hydrogen to dangling bonds and disappearance of the dangling bonds as a result.
Further, as a method of forming a polycrystalline semiconductor film such as a polysilicon film, a technique that includes the following steps is known: dehydrogenation treatment for reducing a hydrogen concentration is performed in advance to an amorphous silicon film so that the surface thereof can be prevented from getting rough, a pulsed excimer laser beam is processed into a linear shape by an optical system, and the dehydrogenated amorphous silicon film is scanned and irradiated with the linear laser beam so as to be crystallized.
The thin film transistor using a polycrystalline semiconductor film has advantages in that its mobility is two or more orders of magnitude greater than that of the thin film transistor using an amorphous semiconductor film and a pixel portion of a display device and peripheral driver circuits thereof can be formed over the same substrate. However, the process is more complex because of crystallization of a semiconductor film, compared to the case of using an amorphous semiconductor film; accordingly, there are problems in that the yield is decreased and the cost is increased.
The present applicant has disclosed an FET (field effect transistor) in which a channel formation region is formed of a semiconductor having a mixture of a crystalline structure and a noncrystalline structure in Patent Document 1 (U.S. Pat. No. 5,591,987).
In addition, as switching elements in image display devices, a thin film transistor using a microcrystalline semiconductor film has been used (Patent Document 2: Japanese Published Patent Application No. H4-242724 and Patent Document 3: Japanese Published Patent Application No. 2005-49832).
As a conventional method of manufacturing the above thin film transistor, a technique is known in which an amorphous silicon film is formed over a gate insulating film, a metal film is formed over the amorphous silicon film, and the metal film is irradiated with a diode laser beam so that the amorphous silicon film is changed into a microcrystalline silicon film (Non-Patent Document 1: Toshiaki Arai, et al., SID '07 Digest, 2007, pp. 1370-1373). According to this method, the metal film formed over the amorphous silicon film is to convert light energy of the diode laser beam into thermal energy, and the metal film needs to be removed later for completion of a thin film transistor. That is, this is a method in which the amorphous silicon film is heated only by conduction heating from the metal film to form the microcrystalline silicon film.
Other than the method of forming a microcrystalline semiconductor film by irradiating amorphous silicon with laser light, there is a method of forming a microcrystalline semiconductor film by a plasma CVD method. As a manufacturing method of a microcrystalline silicon film by a plasma CVD method, an invention is disclosed in which a frequency in a VHF (very high frequency) band that is 30 MHz or higher is used (Patent Document 4: Japanese Patent No. 3201492)